Combustion and emission characteristics of hydrogen‑diesel dual fuel engines

Due to its high energy content and carbonless structure, Hydrogen has attracted researchers’ attention to be used as an alternative fuel in diesel generation which plays a significant role in providing the required energy, specifically in remote and isolated areas. A comprehensive literature review was conducted and found that there was a lack of enough understanding about the combustion mechanism of this fuel in dual fuel operation conditions with different fuel delivery systems. This study largely focused on the investigation of performance, combustion and emission characteristics of hydrogen-diesel dual fuel engines under different hydrogen energy share ratios and engine loads, specifically at low load operation conditions. Furthermore, it also tried to exploit different strategies to improve engine performance in dual-fuel combustion mode.
In the first stage, a numerical model was developed to study the hydrogen-diesel combustion in which hydrogen was introduced into the intake manifold of the engine (hydrogen fumigation). The developed model was first validated against the experimental results available in the literature and then the performance of the engine operating with different hydrogen energy share rations (HES) at low load conditions was examined. It was found that between the studied cases, the case with 45% HES had the lowest performance with a high proportion of hydrogen in the exhaust gaseous. Therefore, this case was chosen to study the effect of oxygen enrichment strategy on performance improvement. In this method, pure oxygen is introduced into the intake manifold and nitrogen is replaced with oxygen. Different percentages of oxygen by volume were examined for this case and it was found that oxygen enrichment led to a significant improvement in engine performance and diesel fuel consumption. However, introducing more oxygen into the cylinder led to a higher in-cylinder temperature which increased the NOx emission level slightly. In order to control this emission and minimize the impact of oxygen enhancement on the emission, the exhaust gas recirculation (EGR) method was applied in the study. The results showed that a combination of oxygen enrichment strategy and EGR was beneficial in emission and performance improvement.
The study was followed by an investigation of the effect of hydrogen injection timing on hydrogen-diesel dual fuel combustion which the gaseous fuel was injected directly into the cylinder. By using this fuel delivery method, hydrogen combustion was more under control compared to hydrogen fumigation. With this purpose, a numerical model was developed and validated via literature experimental results. In this model, a hydrogen injector was embedded into the cylinder head and different injection timings from 140˚ CA (crank angle) before the top dead centre (BTDC) to 20˚ CA BTDC and hence different hydrogen-air stratifications were examined. The results showed that superior performance was viable by using an intermediate hydrogen injection timing (90˚ CA BTDC). This injection timing was selected for the rest of the study which the constant volume combustion strategy (CVCS) was implemented to improve the hydrogen-diesel dual fuel engine performance. In this technique, a piston dwell was applied to the piston motion profile resulting in more proportion of hydrogen combustion was occurred in the period that the piston was positioned at TDC for a longer time. It was found that CVCS significantly improved the brake thermal efficiency and reduced carbon-based emissions.
At the final stage of this research, an experimental study was conducted to investigate the effect of the oxyhydrogen mixture as an additive on diesel generator responsiveness and emission characteristics. In this experiment, the oxyhydrogen mixture was produced by an onboard electrolyser power by the engine battery. The application of an on-board electrolyser was beneficial to address the hydrogen storage problem and safety issues. Three different oxyhydrogen mass flow rates were examined, and the results were compared to the neat diesel operation mode. It was found that introducing this additive made the diesel engine more responsive compared to the neat diesel mode once it experienced a significant step load. Results showed that the frequency was recovered to 50 Hz after a rapid drop in a shorter time. This was attributed to the shorter ignition delay due to the increase in oxygen availability and in-cylinder temperature. In terms of emission levels, it was observed that while the NOx emissions did not change significantly, CO emissions were reduced by 5%. This provided useful information for designing onboard hydrogen technology in engines.